The present invention relates to data transmission systems including a plurality of transmitter/receiver stations, and in particular although not exclusively to such systems in which data signals from any one station are transmitted over a medium common to all stations, the medium being either wired or wireless.
In such systems it is important that the data signals are received at the correct destination and without corruption. At the same time it is often desirable that the data signals are receivable only at selected stations of a system, either to enable the operation of groups of stations within the system as separate networks or, by restricting reception to selected stations within a group, to reduce unnecessary workload on receiver processors which are not required to receive particular data signals. For example, respective groups of stations in adjacent factories may be required to operate as separate networks while utilising a common radio frequency channel, where the area coverages of radio signals being transmitted within the groups may overlap.
One method of checking whether data signals have been received correctly is to use a cyclic redundancy checksum (CRC), where the data to be transmitted is divided into convenient packets or groups of packets, each packet or group being subjected to a mathematical operation known to both the transmitter and the intended receiver of the data so as to arrive at a convenient digital checksum value that is transmitted with the packet or group. A receiver performs the identical mathematical operation on the received data and compares the resultant digital value with that transmitted with the packet or group. If the two checksum values are identical, the receiver knows to a high degree of probability that the data packet or group has been received correctly. If there is any difference, that data packet or group may be arranged to be retransmitted, for example by the absence on the transmission medium of any acknowledgment of correct reception by the intended receiver station.